1. Field of the Invention
This invention relates to motorcycle engines and, more particularly, to a combination carburetor backing plate and crankcase breather that couples an air cleaner to the carburetor and provides a channel for conducting blow-by gases vented from the crankcase into the air cleaner.
2. Background Art
As internal combustion engines wear, the annular spaces between the pistons and the cylinder walls increase. Following ignition of an air-fuel mixture in each cylinder, a part of the gases formed during combustion is forced through the annular space between the piston and the cylinder into the crankcase. These "blow-by" gases, comprising both burned and unburned portions of the fuel mixture, must be vented from the crankcase to the atmosphere to prevent a dangerous buildup of pressure within the crankcase.
Crankcase venting is accomplished in the four cycle, two cylinder, motorcycle engines manufactured by Harley-Davidson, Inc., by a crankcase breather system that conducts blow-by gases from the crankcase to an air cleaner, from which the gases are recirculated back into the engine through the carburetor for burning in the engine cylinders. For each cylinder, there is provided a pair of push rods that extend from the crankcase into the cylinder head for operating the intake and exhaust valves. Each of the push rods is disposed within a hollow push rod tube that communicates with the crankcase. Upper ends of the push rod tubes communicate with a chamber in the middle rocker box of each cylinder head. On each downstroke of the piston in each cylinder, an umbrella valve attached to a center rocker spacer opens, permitting blow-by gases to pass into the middle chamber. From the middle chamber, the blow-by gases are conducted to a second, upper chamber in the rocker box, and thence, via passageways, through a pair of spaced-apart, threaded holes in a right sides of the cylinder heads--the threaded holes serving to receive hollow bolts for securing a stock plastic carburetor backing plate to the cylinder heads. In a stock version of the Harley Davidson crankcase breather system, a stock plastic carburetor backing plate is provided, together with a pair of rubber tubes to conduct the blow-by gases from the hollow bolts to the air filter element.
Prior to our invention, to improve engine performance, purchasers of such motorcycles have usually removed and discarded the stock carburetor backing plate, hollow bolts, and rubber tubes, and installed instead a substitute crankcase breather system as disclosed in U.S. Pat. No. 5,233,967 to Peller. The Peller crankcase breather system included a first tubular member having a first and a second, open-ended cylindrical fitting integrally mounted at opposite ends thereof and communicating with the tube. The fittings were secured to the cylinder heads by means of hollow bolts inserted into threaded holes formed in each cylinder head. Each threaded hole, as described above, communicated with the upper chamber of a cylinder head in the standard manner. The system further included a carburetor backing plate having one side adapted for mounting an air filter and an opposite side adapted for mounting to the cylinder heads by means of mounting brackets and mounting bolts. The system also included a tube having one end attached to a nipple portion of the first fitting and an opposite end attached to an L-shaped nipple threadably received in an opening in the backing plate for communicating with the air cleaner. Blow-by gases exiting the cylinder heads through the threaded holes and hollow bolts were conducted through the first tubular member and through the tube to the air cleaner, and thence recirculated through the engine.
Although it was an improvement over the Harley Davidson stock crankcase breather system, the Peller system had some disadvantages. The multiplicity of parts in the Peller system slowed and complicated installation and removal of the system. The mounting brackets included in the Peller system permitted the carburetor to flex and vibrate, which tended to break the carburetor-to-manifold seal. This resulted in a lean running condition and occasional engine coughing and backfire.
There remains a need, therefore, for a crankcase breather system that requires very few parts to be attached to the exterior of the cylinder heads, is fast and easy to install and remove from a motorcycle engine, and so securely couples an air cleaner to the cylinder heads and carburetor as to substantially eliminate carburetor flexure and vibration during engine operation.